Mathematical Modeling of Brain Activity under Specific Auditory Stimulation.

Understanding the connection between different stimuli and the brain response represents a complex research area. However, the use of mathematical models for this purpose is relatively unexplored. The present study investigates the effects of three different auditory stimuli on cerebral biopotentials by means of mathematical functions. The effects of acoustic stimuli (S1, S2, and S3) on cerebral activity were evaluated by electroencephalographic (EEG) recording on 21 subjects for 20 minutes of stimulation, with a 5-minute period of silence before and after stimulation. For the construction of the mathematical models used for the study of the EEG rhythms, we used the Box-Jenkins methodology. Characteristic mathematical models were obtained for the main frequency bands and were expressed by 2 constant functions, 8 first-degree functions, a second-degree function, a fourth-degree function, 6 recursive functions, and 4 periodic functions. The values obtained for the variance estimator are low, demonstrating that the obtained models are correct. The resulting mathematical models allow us to objectively compare the EEG response to the three stimuli, both between the stimuli itself and between each stimulus and the period before stimulation.

The Influence of Different Auditory Stimuli on Attentiveness and Responsiveness in Road Traffic in Simulated Traffic Situations.

The use of portable media has become an integral part of our increasingly mobile society. The use of digital audio books is also growing steadily in Germany. The connection between the psychological effect of music of different volumes and rhythms and the change in reaction in road traffic with a corresponding increase in risk behavior, especially when driving, has already been proven in previous studies. Only a few studies are available on the effects of listening to radio plays on reaction behavior and concentration in road traffic as well as on risk behavior among pedestrians and cyclists. In the present study, we have investigated the influences of pop music and a radio play on reaction behavior and thus driving ability during the execution of a traffic psychological test series from the "Wiener Test System". The central topic deals with the performance of the test subjects in the individual tests. Conclusions are drawn on the reaction behavior and concentration during participation in road traffic and thus the risk of distraction and possible increased risk of accidents. Studies on the influence of auditory stimuli and their effects on concentration and reaction during participation in traffic are of great interest from the point of view of traffic psychology and occupational medicine, since a reduction in the risk of accidents can increase general traffic safety and lead to a decrease in sick leave and therefore fewer absences from work.

Capacities and neural mechanisms for auditory statistical learning across species.

Statistical learning has been proposed as a possible mechanism by which individuals can become sensitive to the structures of language fundamental for speech perception. Since its description in human infants, statistical learning has been described in human adults and several non-human species as a general process by which animals learn about stimulus-relevant statistics. The neurobiology of statistical learning is beginning to be understood, but many questions remain about the underlying mechanisms. Why is the developing brain particularly sensitive to stimulus and environmental statistics, and what neural processes are engaged in the adult brain to enable learning from statistical regularities in the absence of external reward or instruction? This review will survey the statistical learning abilities of humans and non-human animals with a particular focus on communicative vocalizations. We discuss the neurobiological basis of statistical learning, and specifically what can be learned by exploring this process in both humans and laboratory animals. Finally, we describe advantages of studying vocal communication in rodents as a means to further our understanding of the cortical plasticity mechanisms engaged during statistical learning. We examine the use of rodents in the context of pup retrieval, which is an auditory-based and experience-dependent form of maternal behavior.

Interactive effects of linguistic abstraction and stimulus statistics in the online modulation of neural speech encoding.

Speech processing is highly modulated by context. Prior studies examining frequency-following responses (FFRs), an electrophysiological 'neurophonic' potential that faithfully reflects phase-locked activity from neural ensembles within the auditory network, have demonstrated that stimulus context modulates the integrity of speech encoding. The extent to which context-dependent encoding reflects general auditory properties or interactivities between statistical and higher-level linguistic processes remains unexplored. Our study examined whether speech encoding, as reflected by FFRs, is modulated by abstract phonological relationships between a stimulus and surrounding contexts. FFRs were elicited to a Mandarin rising-tone syllable (/ji-TR/, 'second') randomly presented with other syllables in three contexts from 17 native listeners. In a contrastive context, /ji-TR/ occurred with meaning-contrastive high-level-tone syllables (/ji-H/, 'one'). In an allotone context, TR occurred with dipping-tone syllables /ji-D/, a non-meaning-contrastive variant of /ji-TR/. In a repetitive context, the same /ji-TR/ occurred with other speech tokens of /ji-TR/. Consistent with prior work, neural tracking of /ji-TR/ pitch contour was more faithful in the repetitive condition wherein /ji-TR/ occurred more predictably (p = 1) than in the contrastive condition (p = 0.34). Crucially, in the allotone context, neural tracking of /ji-TR/ was more accurate relative to the contrastive context, despite both having an identical transitional probability (p = 0.34). Mechanistically, the non-meaning-contrastive relationship may have augmented the probability to /ji-TR/ occurrence in the allotone context. Results indicate online interactions between bottom-up and top-down mechanisms, which facilitate speech perception. Such interactivities may predictively fine-tune incoming speech encoding using linguistic and statistical information from prior context.

Improved interaural timing of acoustic nerve stimulation affects sound localization in single-sided deaf cochlear implant users.

The main impairment associated with single-sided deafness (SSD) is the loss of binaural hearing. Currently, the most effective treatment to compensate for this deficit is to supply patients suffering from SSD with a cochlear implant (CI) in the deaf ear. With this approach binaural hearing abilities can be restored to a certain extent, which is expressed in an improvement in such patients with regard to sound source localization and speech comprehension in noise after receipt of a CI. However, binaural performance of these listeners does not reach the level of normal-hearing listeners. One of the reasons for this might be that the electrical stimulation via CI and the physiological stimulation via the intact ear are not synchronized: the CI transmits the information to the auditory nerve with different timing than does the intact inner ear. As a result, there is a timing mismatch of the information transfer between the left and the right side, which may account for the limited binaural performance. The effective mismatch in timing depends on the CI system because of different stimulation strategies implemented in devices from different manufacturers. For the particular CI device used in this study (MED-EL Mi1000/Mi1200) electrical stimulation led to faster activation of the auditory nerve than natural for a wide frequency range. In particular, electrical stimulation was about 1 to up to 2 ms ahead of time for frequencies above 1.5 kHz. Hence, it was hypothesized that information transfer between the left and the right ear can be tuned by delaying the CI signal. The goal of the present study was to investigate whether such a delay in the CI signal affects binaural performance of CI users with SSD. For this purpose, sound source localization and speech perception in noise were tested in a sample of 12 CI users with SSD (mean age 51 ±â€¯12 years). The tests were performed for four different delay times of the CI signal applied spontaneously (0.5, 1, 2 and 4 ms) and for the base line condition "no delay" in the CI signal (i.e. everyday use). It was found that delaying the signal had a significant impact on sound source localization. Speech perception in noise was affected, but less pronounced than was sound localization. Regarding sound source localization, a signal delay of 1 ms applied to this particular CI device produced the best performance in our patients. It is concluded that improving the synchronisation between the CI-transferred signal and the naturally transferred signal could increase binaural hearing performance in CI users with SSD.

Noise in the neonatal intensive care unit: a new approach to examining acoustic events.

INTRODUCTION: Environmental noise is associated with negative developmental outcomes for infants treated in the neonatal intensive care unit (NICU). The existing noise level recommendations are outdated, with current studies showing that these standards are universally unattainable in the modern NICU environment. STUDY AIM: This study sought to identify the types, rate, and levels of acoustic events that occur in the NICU and their potential effects on infant physiologic state. MATERIALS AND METHODS: Dosimeters were used to record the acoustic environment in open and private room settings of a large hospital NICU. Heart and respiratory rate data of three infants located near the dosimeters were obtained. Infant physiologic data measured at time points when there was a marked increase in sound levels were compared to data measured at time points when the acoustic levels were steady. RESULTS: All recorded sound levels exceeded the recommended noise level of 45 decibels, A-weighted (dBA). The 4-h Leq of the open-pod environment was 58.1 dBA, while the private room was 54.7 dBA. The average level of acoustic events was 11-14 dB higher than the background noise. The occurrence of transient events was 600% greater in the open room when compared to the private room. While correlations between acoustic events and infant physiologic state could not be established due to the extreme variability of infant state, a few trends were visible. Increasing the number of data points to overcome the extreme physiologic variability of medically fragile neonates would not be feasible or cost-effective in this environment. CONCLUSION: NICU noise level recommendations need to be modified with an emphasis placed on reducing acoustic events that disrupt infant state. The goal of all future standards should be to optimize infant neurodevelopmental outcomes.

Stimulus Statistics Change Sounds from Near-Indiscriminable to Hyperdiscriminable.

Objects and events in the sensory environment are generally predictable, making most of the energy impinging upon sensory transducers redundant. Given this fact, efficient sensory systems should detect, extract, and exploit predictability in order to optimize sensitivity to less predictable inputs that are, by definition, more informative. Not only are perceptual systems sensitive to changes in physical stimulus properties, but growing evidence reveals sensitivity both to relative predictability of stimuli and to co-occurrence of stimulus attributes within stimuli. Recent results revealed that auditory perception rapidly reorganizes to efficiently capture covariance among stimulus attributes. Acoustic properties per se were perceptually abandoned, and sounds were instead processed relative to patterns of co-occurrence. Here, we show that listeners' ability to distinguish sounds from one another is driven primarily by the extent to which they are consistent or inconsistent with patterns of covariation among stimulus attributes and, to a lesser extent, whether they are heard frequently or infrequently. When sounds were heard frequently and deviated minimally from the prevailing pattern of covariance among attributes, they were poorly discriminated from one another. In stark contrast, when sounds were heard rarely and markedly violated the pattern of covariance, they became hyperdiscriminable with discrimination performance beyond apparent limits of the auditory system. Plausible cortical candidates underlying these dramatic changes in perceptual organization are discussed. These findings support efficient coding of stimulus statistical structure as a model for both perceptual and neural organization.

An evaluation of hearing preservation outcomes in routine cochlear implant care: Implications for candidacy.

INTRODUCTION: A retrospective evaluation of pre- and post-operative audiometric data at a single large UK cochlear implant centre over 25 years was undertaken. METHODS: Analysis of pre-operative hearing levels showed that there was a modest but significant reduction in average pre-operative hearing thresholds among patients referred between 1990 and 2015, particularly in the low frequencies. OUTCOMES: The proportion of those referred who would meet widely-accepted candidacy criteria for electro-acoustic stimulation (EAS) grew significantly over time but in the period 2011-2015 just 9% of those referred had sufficient residual hearing for EAS. On average, implant recipients lost 20 dB hearing at frequencies ≤1000 Hz as a result of the surgery. CONCLUSIONS: The findings suggest that hearing preservation is now widely achievable, and that both candidacy criteria and referrer education should take into account potential EAS benefit.

Adult hearing-aid users with cochlear dead regions restricted to high frequencies: Implications for amplification.

OBJECTIVES: Cochlear dead regions (DR) are common in adult hearing-aid users, but are usually restricted to high frequencies. The aim was to determine the benefit of high-frequency amplification for ears with and without high-frequency DRs. DESIGN: Participants were fitted with the study hearing aid and tested under four conditions: unfiltered (NAL-NL2 prescription), and low-pass filtered at 1.5, 2, and 3 kHz. VCV stimuli were presented at 65 dB (A) in quiet and in 20-talker babble at a signal-to-babble ratio of 0 dB. STUDY SAMPLE: Experienced adult hearing-aid users: one group of 18 with a DR edge frequency above 1.5 kHz, and a group of 18 matched controls. RESULTS: Overall performance was best in the unfiltered condition. There was no significant difference in mean performance between the two groups when tested in quiet. However, the DR group obtained less benefit from high-frequency amplification when tested in babble: the mean difference between the unfiltered and 3-kHz filtered condition was 6% and 13% for the DR group and controls, respectively. CONCLUSIONS: In adults with a moderate hearing loss and a restricted DR, speech recognition was always best in the unfiltered condition, although mean performance in babble was lower for the DR group.

Efeitos do treinamento auditivo computadorizado em crianças com distúrbio do processamento auditivo e sistema fonológico típico e atípico / Effects of computerized auditory training in children with auditory processing disorder and typical and atypical phonological system

RESUMO Objetivo: Investigar os efeitos do treinamento auditivo computadorizado (TAC) por meio da análise do desempenho em testes comportamentais e da Escala de Funcionamento Auditivo (SAB), em crianças com distúrbio do processamento auditivo (DPA) e sistema fonológico típico e atípico. Métodos: Participaram 14 crianças com DPA, sete crianças com DPA e aquisição fonológica típica (G1) e sete com DPA e aquisição fonológica atípica (G2). Foi realizada a avaliação comportamental do processamento auditivo com o Teste de Detecção de Intervalo Aleatório (RGDT), Teste de Inteligibilidade Pediátrica (PSI), Teste Dicótico Não Verbal (TDNV) e Escala de Funcionamento Auditivo (SAB). Para composição dos grupos realizou-se a Avaliação Fonológica da Criança. A intervenção terapêutica foi realizada por meio do software Escuta Ativa, com 12 sessões, duas vezes por semana. Utilizou-se testes estatísticos adequados. Resultados: No desempenho nos testes comportamentais do processamento auditivo (PA), pré-TAC e pós-TAC, o resultado foi significativo no RGDT e TDNV, etapa de escuta direcionada à esquerda em ambos os grupos e TDNV à direita apenas no G2. Houve correlação positiva entre SAB e algumas condições de escuta do teste PSI em ambos os grupos, tanto pré-TAC quanto pós-TAC, além de correlação negativa entre a SAB e o teste RGDT pós-TAC, no grupo com aquisição fonológica típica e DPA. A SAB mostrou-se um instrumento útil para mensurar o efeito do TAC. Conclusão: O TAC mostrou-se eficaz para melhorar/adequar os processos gnósicos auditivos em escolares dos grupos estudados. Houve correlação entre os testes comportamentais e o escore na SAB.

ABSTRACT Purpose: To investigate the effects of computerized auditory training (CAT) through performance analysis of behavioral tests and Scale of Auditory Behaviors (SAB), in children with Auditory Processing Disorder (APD) and typical and atypical phonological system. Methods: Forteen children with APD, seven children with APD and typical phonological acquisition (G1) and seven with APD and atypical phonological acquisition (G2) participated. It was performed an auditory processing behavioral evaluation with Random Gap Detection Test (RGDT), Pediatric Speech Intelligibility test (PSI), Nonverbal Dichotic Test (NVDT) and SAB. For composition of the groups it was performed a Child Phonological Assessment. The therapeutic intervention was performed using the Escuta Ativa software, with 12 sessions held twice a week. Appropriate statistical tests were used. Results: In the performance in behavioral tests of auditory processing (AP), pre-and post-CAT, there was a significant result in RGDT and NVDT, hearing stage directed to the left in both groups and NVDT to the right only in G2. There was a positive correlation between SAB and some listening conditions of the PSI test in both groups, including pre and post-CAT. Moreover, there was a negative correlation between SAB and the RGDT test post-CAT in the group with typical phonological acquisition and APD. SAB proved to be a useful tool to measure the effect of CAT. Conclusion: CAT was effective to improve/adapt the gnosis auditory processes in the studied school groups. There was a correlation between behavioral tests and the score in the SAB.

Desempenho de crianças com distúrbio de leitura após o treino auditivo / Performance of children with reading difficulties after auditory training

Objetivo : Verificar o desempenho de leitura e da habilidade auditiva de resolução temporal em crianças com dificuldades de leitura, após o treino auditivo. Métodos : Participaram deste estudo 20 crianças na faixa etária de 8 anos, sendo dez escolares com dificuldades de leitura (Grupo Estudo), e dez escolares sem dificuldades escolares (Grupo Controle). Todos os indivíduos foram submetidos à avaliação inicial e reavaliação da resolução temporal auditiva (Padrão de Frequência, Padrão de Duração e Gap in noise (GIN) e de leitura (Protocolo de Leitura Clínica e Teste de Consciência Fonológica - CONFIAS). O Grupo Estudo foi submetido a oito sessões de treinamento auditivo. Resultados : Verificou-se que o Grupo Controle obteve maior desempenho em todos os testes aplicados, quando comparado ao Grupo Estudo, na avaliação inicial e na reavaliação. A diferença do desempenho antes e depois do treinamento auditivo foi significativa para quase todas as tarefas no Grupo Estudo, exceto para a consciência fonológica. Conclusão : O treino auditivo foi efetivo para a melhora de desempenho nas habilidades auditivas temporais e de leitura em crianças que apresentam dificuldades de leitura. .

Purpose : To evaluate reading performance and auditory temporal resolution in children with reading difficulties after auditory training. Methods : Twenty 8 year- old children, 10 with reading difficulties (Study Group) and 10 without reading difficulties (Control Group) were assessed. All subjects participated in an initial assessment and reassessment of auditory temporal resolution (Frequency Pattern, Duration Pattern, and Gap in Noise (GIN) Tests) and reading performance (Clinical Reading Protocol and Phonological Awareness Test - CONFIAS). The Study Group was submitted to eight auditory training sessions. Results : The Control Group showed superior performance on all tests compared to the Study Group at both assessment and reassessment. The difference in performance before and after training was statistically significant for almost all tasks in the Study Group, except for phonological awareness. Conclusion : The auditory training proved effective for improving performance on auditory temporal and reading tasks in children with reading difficulties. .

Sensation of agency and perception of temporal order.

After adaptation to a fixed temporal delay between actions and their sensory consequences, stimuli delivered during the delay are perceived to occur prior to actions. Temporal judgments are also influenced by the sensation of agency (experience of causing our own actions and their sensory consequences). Sensory consequences of voluntary actions are perceived to occur earlier in time than those of involuntary actions. However, it is unclear whether temporal order illusions influence the sensation of agency. Thus, we tested how the illusionary reversal of motor actions and sound events affect the sensation of agency. We observed an absence of the sensation of agency in the auditory modality in a condition in which sounds were falsely perceived as preceding motor acts relative to the perceived temporal order in the control condition. This finding suggests a strong association between the sensation of agency and the temporal order perception of actions and their consequences.

Unilateral versus bilateral upper limb training after stroke: the Upper Limb Training After Stroke clinical trial.

BACKGROUND AND PURPOSE: Unilateral and bilateral training protocols for upper limb rehabilitation after stroke represent conceptually contrasting approaches with the same ultimate goal. In a randomized controlled trial, we compared the merits of modified constraint-induced movement therapy, modified bilateral arm training with rhythmic auditory cueing, and a dose-matched conventional treatment. Modified constraint-induced movement therapy and modified bilateral arm training with rhythmic auditory cueing targeted wrist and finger extensors, given their importance for functional recovery. We hypothesized that modified constraint-induced movement therapy and modified bilateral arm training with rhythmic auditory cueing are superior to dose-matched conventional treatment. METHODS: Sixty patients, between 1 to 6 months after stroke, were randomized over 3 intervention groups. The primary outcome measure was the Action Research Arm test, which was conducted before, directly after, and 6 weeks after intervention. RESULTS: Although all groups demonstrated significant improvement on the Action Research Arm test after intervention, which persisted at 6 weeks follow-up, no significant differences in change scores on the Action Research Arm test were found between groups postintervention and at follow-up. CONCLUSIONS: Modified constraint-induced movement therapy and modified bilateral arm training with rhythmic auditory cueing are not superior to dose-matched conventional treatment or each other in improving upper limb motor function 1 to 6 months after stroke. CLINICAL TRIAL REGISTRATION URL: http://www.trialregister.nl. Unique identifier: NTR1665.

Paediatric occupational therapists' use of sound-based interventions: a survey study.

Sound-based interventions (SBIs) involve listening to psychoacoustically modified music as a means for promoting new neural pathways in individuals with neurological dysfunction. Multiple forms of SBIs exist and are being used by healthcare professionals despite the lack of evidence to support their efficacy. The purpose of this study was to describe how occupational therapists (OTs) are using SBIs as an intervention in paediatric practice. An online survey was conducted using SurveyMonkey® and was completed by OTs describing their use of SBIs with paediatric clients. Paediatric OTs are using SBIs in a variety of practice settings, with children with a variety of medical and developmental conditions, and primarily use a subjective assessment process for determining effectiveness. Most referrals for SBIs come from other OTs and parents, and reimbursement primarily is achieved through private-pay mechanisms. This study had a small sample size (n = 74) and an estimated response rate of 14.7%, so generalization to occupational therapy practice in the United States is not possible. Further research is needed to identify the internal and external influences on the selection of SBIs as an OT intervention in paediatric practice.

Development of a loudness normalisation strategy for combined cochlear implant and acoustic stimulation.

Users of a cochlear implant together with a hearing aid in the non-implanted ear currently use devices that were developed separately and are often fitted separately. This results in very different growth of loudness with level in the two ears, potentially leading to decreased wearing comfort and suboptimal perception of interaural level differences. A loudness equalisation strategy, named 'SCORE bimodal', is proposed. It equalises loudness growth for the two modalities using existing models of loudness for acoustic and electric stimulation, and is suitable for implementation in wearable devices. Loudness balancing experiments were performed with six bimodal listeners to validate the strategy. In a first set of experiments, the function of each loudness model used was validated by balancing the loudness of four harmonic complexes of different bandwidths, ranging from 200 Hz to 1000 Hz, separately for each ear. Both the electric and acoustic loudness models predicted the data well. In a second set of experiments, binaural balancing was done for the same stimuli. It was found that SCORE significantly improved binaural balance.

Incorporating cross-modal statistics in the development and maintenance of multisensory integration.

Development of multisensory integration capabilities in superior colliculus (SC) neurons was examined in cats whose visual-auditory experience was restricted to a circumscribed period during early life (postnatal day 30-8 months). Animals were periodically exposed to visual and auditory stimuli appearing either randomly in space and time, or always in spatiotemporal concordance. At all other times animals were maintained in darkness. Physiological testing was initiated at ∼2 years of age. Exposure to random visual and auditory stimuli proved insufficient to spur maturation of the ability to integrate cross-modal stimuli, but exposure to spatiotemporally concordant cross-modal stimuli was highly effective. The multisensory integration capabilities of neurons in the latter group resembled those of normal animals and were retained for >16 months in the absence of subsequent visual-auditory experience. Furthermore, the neurons were capable of integrating stimuli having physical properties differing significantly from those in the exposure set. These observations suggest that acquiring the rudiments of multisensory integration requires little more than exposure to consistent relationships between the modality-specific components of a cross-modal event, and that continued experience with such events is not necessary for their maintenance. Apparently, the statistics of cross-modal experience early in life define the spatial and temporal filters that determine whether the components of cross-modal stimuli are to be integrated or treated as independent events, a crucial developmental process that determines the spatial and temporal rules by which cross-modal stimuli are integrated to enhance both sensory salience and the likelihood of eliciting an SC-mediated motor response.

A real-time detector system for precise timing of audiovisual stimuli.

The successful recording of neurophysiologic signals, such as event-related potentials (ERPs) or event-related magnetic fields (ERFs), relies on precise information of stimulus presentation times. We have developed an accurate and flexible audiovisual sensor solution operating in real-time for on-line use in both auditory and visual ERP and ERF paradigms. The sensor functions independently of the used audio or video stimulus presentation tools or signal acquisition system. The sensor solution consists of two independent sensors; one for sound and one for light. The microcontroller-based audio sensor incorporates a novel approach to the detection of natural sounds such as multipart audio stimuli, using an adjustable dead time. This aids in producing exact markers for complex auditory stimuli and reduces the number of false detections. The analog photosensor circuit detects changes in light intensity on the screen and produces a marker for changes exceeding a threshold. The microcontroller software for the audio sensor is free and open source, allowing other researchers to customise the sensor for use in specific auditory ERP/ERF paradigms. The hardware schematics and software for the audiovisual sensor are freely available from the webpage of the authors' lab.

Determination of the loudness dependence of auditory evoked potentials: single-electrode estimation versus dipole source analysis.

OBJECTIVE: The loudness dependence of auditory evoked potentials (LDAEP) has been described as a measure of central serotonergic activity. Single-electrode estimation and dipole source analysis (DSA) are the most utilized methods for the estimation of LDAEP. To date, it is assumed that both methods are equally reliable. Nevertheless, according to our knowledge, the advantage of either method has not yet been shown directly. The aim of our study was to compare single-electrode estimation and dipole source analysis in the determination of the LDAEP. METHODS: Tones of five different intensities were presented binaurally to 10 healthy volunteers. Amplitudes of N1/P2 and LDAEP were determined at the central electrode site referenced to average and to linked mastoids and with DSA in the left and the right hemispheres. Scores were normalized (z-scores), compared, and correlated. RESULTS: Contrary to our expectations, we found a significant difference between scores obtained with single-electrode estimation and with DSA. CONCLUSION: The difference may be caused by confounding activation of a frontal source in the single-electrode method. The single-electrode approach cannot be equated with DSA in the determination of the LDAEP. This should be considered when comparing the results of different LDAEP studies using only one of these methods.

Dynamic range adaptation to sound level statistics in the auditory nerve.

The auditory system operates over a vast range of sound pressure levels (100-120 dB) with nearly constant discrimination ability across most of the range, well exceeding the dynamic range of most auditory neurons (20-40 dB). Dean et al. (2005) have reported that the dynamic range of midbrain auditory neurons adapts to the distribution of sound levels in a continuous, dynamic stimulus by shifting toward the most frequently occurring level. Here, we show that dynamic range adaptation, distinct from classic firing rate adaptation, also occurs in primary auditory neurons in anesthetized cats for tone and noise stimuli. Specifically, the range of sound levels over which firing rates of auditory nerve (AN) fibers grows rapidly with level shifts nearly linearly with the most probable levels in a dynamic sound stimulus. This dynamic range adaptation was observed for fibers with all characteristic frequencies and spontaneous discharge rates. As in the midbrain, dynamic range adaptation improved the precision of level coding by the AN fiber population for the prevailing sound levels in the stimulus. However, dynamic range adaptation in the AN was weaker than in the midbrain and not sufficient (0.25 dB/dB, on average, for broadband noise) to prevent a significant degradation of the precision of level coding by the AN population above 60 dB SPL. These findings suggest that adaptive processing of sound levels first occurs in the auditory periphery and is enhanced along the auditory pathway.

Application of frequency modulated chirp stimuli for rapid and sensitive ABR measurements in the rat.

Rodents have proven to be a useful model system to screen genes, ototoxic compounds and sound exposure protocols that may play a role in hearing loss. High-throughput screening depends upon a rapid and reliable functional assay for hearing loss. This study describes the use of a frequency modulated (FM) chirp stimulus as an alternative to the click to derive a rapid assessment of auditory brainstem response (ABR) threshold in the rodent. We designed a rising frequency A-chirp based upon the spatial mapping of preferred frequency along the rat basilar membrane to provide a more synchronous and equipotent input across the length of the cochlea. We observed that the ABR wave I and wave IV amplitudes evoked by the A-chirp were significantly greater than the click and that A-chirp minimum response thresholds were lower than the click. Subsequent analyses compared the efficacy of the A-chirp to linear, time-reversed and amplitude-reversed chirps and confirmed that the A-chirp was most effective chirp configuration. These data suggest that the A-chirp may be optimally suited as a single screening broad-frequency stimulus for rapid ABR threshold estimations in the rodent and could serve to complement more detailed frequency-specific physiologic and behavioral estimates of hearing threshold.